Conventionally, as a gaseous fuel tank module of this kind, there has been known a gaseous fuel tank module having a support frame member and a gaseous fuel tank fixed to the support frame member horizontally. In this case, as the gaseous fuel tank, there has been used a gaseous fuel tank having a cylindrical tank main body and a neck portion which allows a gaseous fuel inlet and outlet valve device to be mounted in such a manner as to protrude from an axial end portion of the tank main body (for example, see JP-A-2002-106787).